Radiography for medical and other purposes has been available since 1896, shortly after the discovery of x-rays by Roentgen. In medical radiography, x-ray film is placed inside a light-tight cassette, where it lays in close contact with one or two x-ray intensifying screens. Dual-screen systems are most common for general radiographic work, and so these systems will be described.
The x-ray film is generally composed of a transparent, thin plastic base with two photographic emulsions, one coated onto each side. When placed inside the cassette, the two x-ray intensifying screens make very tight physical contact with the film. The cassette is placed behind the anatomical area of the patient that is to be radiographed, and the x-rays are turned on briefly. The x-rays pass through the patient and an x-ray shadow is cast onto the cassette which shadow has a profile reflective of the anatomical profile within the radiographed area.
The photographic emulsion is a thin layer composed of silver bromide, which is not a very efficient absorber of x-rays. The purpose of the intensifying screens is to absorb the x-rays. Such absorption within each screen causes radiation emissions in the ultraviolet, visible or infrared region of the electromagnetic spectrum. This emitted radiation ("light") then exposes the photographic emulsion. The exposed film is developed chemically, and the radiographic image is rendered visible. Although the indirect exposure of the photographic emulsion by the light emitted by the screen reduces some of the spatial resolution in the image, this loss is acceptable because the introduction of the screen results in a dramatic reduction in radiation dose to the patient. Screens are virtually always used in diagnostic radiography.
X-ray intensifying screens are intrinsically linear, over a wide dynamic range. This means that the light emitted by the screen increases in direct, linear, proportion to the amount of x-rays that strike the screen (x-rays of the same energy distribution). Unfortunately, the dynamic range of the film cannot match that of the screen. Dynamic range in radiological imaging is also called latitude.
The characteristic curve of a typical screen-film system is shown in FIG. 1, also referred to as an Hurter-Drifield curve or H & D curve. The optical density of radiographic film ranges from about 0.13 (which reflects primarily attenuation of light by the film base plus a minimal amount of fog), to a maximum of about 3.1 to 3.6 or so. The characteristic curve for typical film used in radiographic systems is plotted as the optical density versus radiation exposure to the cassette.
Low exposures result in images that are very light, and have less contrast, than images exposed towards the center of the characteristic curve. Exposures that are about midrange in the characteristic curve result usually in the best looking images, because this is where the contrast of the film is the greatest. High exposures result in images that are too dark to be useful, and these often need to be repeated.
Contrast can be calculated as the slope of the characteristic curve, and this is plotted in FIG. 1 also. With proper exposure, the radiographic image can be well visualized when the film is placed on a view box which provides back illumination of the film. This is the normal viewing situation in radiology.
The characteristic curve of film imposes a compromise for designers of screen-film systems. For screen-film systems which have a large dynamic range (wide latitude in radiology vernacular), the contrast of the image is necessarily reduced. On the other hand, for high contrast screen-film systems, the latitude or dynamic range of the system is reduced. Because of these two constraints, wide latitude systems are generally used in imaging procedures where a large dynamic range needs to be imaged (e.g. chest radiography), and high contrast, reduced-latitude systems are used in imaging applications where only a small latitude needs to be accommodated (e.g. bone radiography).
In conventional screen-film radiography, the intensifying screen is coupled directly to a film. Many efforts have focused on using an electronic imaging device, such as a charge coupled display (CCD) camera, that is positioned to detect the light emitted from the intensifying screen. These scenarios typically have the CCD camera optically or fiber-optically coupled to the light emitted from a screen when struck by x-rays. There is a well known, fundamental problem in this design that arises due to the quantum sink, i.e., loss of light, that the optical coupling stage poses in the system.
Consequently, a need still exists for a screen-film system having a large dynamic range and high contrast and for a screen-film system which avoids the quantum sink problem. The invention achieves such a system through a unique combination of x-ray phosphors, each with specific response characteristics such as the quenchability of light emitted in response to x-ray irradiation. The quenchable phosphor can be used in a design that will not only provide large dynamic range and high contrast, but which will also overcome the quantum sink problems associated with the optical stage in digital radiographic systems. In relation to the description of the invention herein, x-ray phosphors have been catagorized into several classes.
Three classes of x-ray phosphors are envisioned as being combined within the scope of the invention. The general characteristics of each class are now described and the terminology is defined:
Class 1 phosphors are quenchable. They emit light of a particular wavelength or wavelengths when struck by x-rays in a quasi-linear fashion, however when simultaneously struck by other radiations (not x-rays) of a specific wavelength or wavelengths (e.g. infrared), their light emission is reduced. For example, place a class 1 phosphor into a beam of constant-intensity x-rays. It will emit light of particular wavelength(s), say green light. If a source of (for example) infrared radiation is brought near the class 1 phosphor, its emission of green light will be reduced, even if the incident x-ray fluence rate (photons/cm.sup.2 /second) remains constant. The infrared radiation, in this example, quenches some fraction of the class 1 phosphor's characteristic emission. The radiation which causes the class 1 phosphor to reduce its emission is called the quenching radiation. In the above example, infrared radiation was discussed, but quenching radiation can be of any spectral composition outside the x-ray region. In addition, the class 1 phosphor emits radiation of wavelengths that are usable, meaning that these wavelengths are capable of exposing the film emulsion. Film emulsion can be designed to be sensitive to the wavelengths emitted by particular phosphors. The techniques for designing such emulsion sensitivities are known.
Class 2 phosphors emit radiation when struck by x-rays, in a quasi-linear fashion. The class 2 phosphor emits radiation which is of the correct spectral composition such that it quenches the emission of the class 1 phosphor. In other words, the class 2 phosphor emits quenching radiation when struck by x-rays. The emission of the class 2 phosphor is unusable, meaning that the film emulsion is relatively insensitive to these wavelengths.
A class 3 phosphor, in the context of the present discussion, is any phosphor which is not quenchable, does not emit quenching radiation, and emits an usable, i.e., emulsion sensitive, emission.
To summarize the characteristics of the three classes of phosphors defined here, the class 1 phosphor is a quenchable phosphor, which emits usable, non-quenching radiation. Class 2 and class 3 phosphors are conventional (non-quenchable), linear x-ray phosphors, which emit light in proportion to the amount of x-rays incident upon them. Class 2 phosphors emit radiations which are capable of quenching the emission of the class 1 phosphor, but are unusable in exposing the film, whereas the class 3 phosphor emission does not have quenching properties, but is usable in exposing the film.